The present disclosure relates to the use of optical coherence tomography for the phase-sensitive detection of an acoustic vibrations. In particular, the present disclosure relates to phase-sensitive optical coherence tomographic vibrography and optical coherence tomographic elastography.
In phase-sensitive optical coherence tomographic (PS-OCT) vibrography, the sinusoidal motion of a structure located at a particular pixel in the image produces a sinusoidal phase variation between successive A-lines produced by taking the discrete Fourier transform (DFT) of the sampled interferogram (in swept-source OCT) and the sampled spectrum (in spectral domain OCT). In the absence of noise, the measured phase at each vibrating pixel is characterized by a sinusoid.
In real systems, however, there are two major sources of noise that contaminate the signal. First, random broadband noise due to shot noise and residual intensity noise is present in the passband and must be averaged down to an acceptably low level in order to recover the vibratory response. Second, motion noise caused by motion of the subject relative to the OCT optics caused, for example, by breathing and heartbeat, produce very large amplitude and low frequency phase noise that dwarfs the vibrations of interest. For example, in the case of middle ear imaging, the head moves about a millimeter every time the heart beats, but the vibration of the middle ear structures in response to sound are as low as a few nanometers—hundreds of thousands times smaller. Such motion noise presents challenges in signal processing, often leading to very long signal acquisition times that conflict with clinical requirements.